The present invention relates to a magnetic drive unit including a voice coil and a magnetic circuit for moving a driver member such as an objective lens in an optical pickup in small intervals, and more particularly to a magnetic drive unit capable of moving the voice coil linearly in small intervals.
Recently available digital audio disc (DAD) players have an optical pickup for reading information recorded on a disc. The optical pickup has a magnetic drive unit for keeping an objective lens a constant distance from the disc at all times to focus a detecting beam spot on the information-recorded surface in the disc. FIG. 1 of the accompanying drawings is a side elevational view, partly in cross section, of a conventional magnetic drive unit 11'. The magnetic drive unit 11' operates on the same principle as that of an audie loudspeaker. The magnetic drive unit 11' includes a magnet 11b disposed centrally in a yoke 11c and a pole piece 11d bonded to an upper surface of the magnet 11b. The pole piece 11d and an upper flange 11e of the yoke 11c which confronts the pole piece 11d jointly define therebetween a magnetic gap 14 in which a voice coil 11'a is disposed. The voice coil 11'a supports thereon a driver member comprising an objective lens 8 held in confronting relation to the disc, which is denoted at 1. The objective lens 8 is moved vertically through small intervals by varying a current flowing through the voice coil 11'a. Another magnetic drive unit 12' with the yoke 11c serving as a drive member has a voice coil 12'a. The objective lens 8 can be moved horizontally in small intervals by varying a current flowing through the voice coil 12'a. A light beam emitted from an optical detector 15 comprising such as a laser diode is reflected by a prism 7 and focused by the objective lens 8 onto the information-recorded surface in the disc 1. The beam spot on the information-recorded surface in the disc 1 can be focused thereon through small movements of the objective lens 8 driven by the magnetic drive units 11', 12', and adjusted thereby to follow the tracks on the disc 1 accurately. In the field of the optical pickup, the magnetic drive unit 11' is called a "focusing servomechanism" and the magnetic drive unit 12' is called a "tracking servomechansim".
The voice coil 11'a is normally constructed of windings of a copper wire. Since the copper wire is of a round cross section, the coil windings have a poor space factor. The space factor is particularly low where the coil has a plurality of winding layers. To cope with this, there has been proposed to construct the voice coil 11'a of windings of a wire having a rectangularly cross section which gives a better space factor. However, when such a wire is wound, the radially outward portion of the coil is subjected to a tensile stress, while the radially inward portion undergoes a compressive stress, with the result that the coil will be distorted during the manufacturing process. Another proposal is a printed coil which however is required to be produced in a complex manufacturing process and hence is costly to fabricate.
The voice coil 11'a has a uniform winding density. The magnetic field generated in the magnetic gap 14 between the flange 11e of the yoke 11c and the pole piece 11d has a high flux density in its central area, the flux density becoming progressively lower in a direction away from the central area. Accordingly, the force acting on the voice coil 11'a tends to vary as the voice coil 11'a is moved in the magnetic gap 14.